We propose an HTS multiplex screening project to use small molecules to probe a complex, highly conserved, biological pathway. The project takes advantage of the yeast GFP strain collection to probe the TOR pathway, a pathway with therapeutic implications in man [1]. The target of rapamycin, TOR, is an essential ser/thr protein kinase that functions in two distinct multiprotein complexes, TOR complexes 1 and 2. The structure and functions of these complexes have been conserved from yeast to man. TOR complex 1 is inhibited by rapamycin and is thought to couple growth cues to cellular metabolism;TOR complex 2 is not inhibited by rapamycin and appears to regulate spatial aspects of growth, such as cell polarity [2, 3]. Rapamycin, an antifungal compound isolated from a bacterium found in soil on the island of Rapa Nui [4], was the drug originally used to characterize the TOR pathway in budding yeast (S. cerevisiae) by identifying mutants that were rapamycin sensitive or resistant [5]. Rapamycin acts as a cytostatic agent in fungi, arresting cells in a G0-like state. As a drug it is widely used as an immunosuppressant and rapamycin and derivatives of this macrocyclic lactone are being evaluated for a number of clinical applications. Because of its pleiotropic effects, rapamycin is thought of as a "dirty drug" in the pharmaceutical industry. To identify more specific TOR inhibitors and activators, we propose a cell-based multiplex high throughput flow cytometry assay to screen the MLSMR to define chemicals that target specific proteins in the TOR pathway. A chemical screen should yield small molecules that modulate specific branches of this pathway and these molecules may have significant therapeutic potential and fewer side effects. We will screen the yeast GFP-fusion strain set (4,159 strains) [6] prior to and post-rapamycin treatment in multiplex format because the fusion library is highly amenable to fluorescence-based flow cytometric readout. Change in fluorescence indicates that a chemical has affected production of the target GFP-fusion protein(s). Chemicals we identify that mimic or inhibit rapamycin activation will likely have targets in other organisms because TOR pathway components are highly conserved. We have already successfully screened this collection under two different growth conditions and have the capacity to identify chemicals affecting one or more than one of the multiplexed targets. We will conduct primary screens to detect both agonists and antagonists of the TOR pathway. As secondary screens, we will: 1) compare the impact of the novel molecules on protein expression in the GFP collection and 2) evaluate the ability of the small molecules to impact the cytostatic potential of rapamycin through analysis of cell cycle and/or growth arrest. Expected Results: The screen in the absence of rapamycin will reveal molecules that mimic rapamycin or otherwise impact the TOR pathway. The screen in the presence of rapamycin will reveal molecules that antagonize rapamycin. PUBLIC HEALTH RELEVANCE: We propose an HTS multiplex screening project to use small molecules to probe a complex, highly conserved, biological pathway. The project takes advantage of the yeast GFP strain collection to probe the TOR - target of rapamycin - pathway, a pathway with therapeutic implications in man.